Denitrifying Bioreactors—An Approach for Reducing Nitrate Loads
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G Model ECOENG-1647; No. of Pages 12 ARTICLE IN PRESS Ecological Engineering xxx (2010) xxx–xxx Contents lists available at ScienceDirect Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng Review Denitrifying bioreactors—An approach for reducing nitrate loads to receiving waters Louis A. Schipper a,∗, Will D. Robertson b, Arthur J. Gold c, Dan B. Jaynes d, Stewart C. Cameron e a Department of Earth and Ocean Sciences, University of Waikato, Private Bag 3105, Hamilton, New Zealand b Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, ON N2L 3G1, Canada c Department of Natural Resources Science, University of Rhode Island, Kingston, RI 02881, USA d National Laboratory for Agriculture and the Environment, USDA-ARS, 2110 University Blvd, Ames, IA 50011-3120, USA e GNS Science, Private Bag 2000, Taupo, New Zealand article info abstract Article history: Low-cost and simple technologies are needed to reduce watershed export of excess nitrogen to sensitive Received 12 November 2009 aquatic ecosystems. Denitrifying bioreactors are an approach where solid carbon substrates are added Received in revised form 9 March 2010 into the flow path of contaminated water. These carbon (C) substrates (often fragmented wood-products) Accepted 3 April 2010 act as a C and energy source to support denitrification; the conversion of nitrate (NO −) to nitrogen gases. Available online xxx 3 Here, we summarize the different designs of denitrifying bioreactors that use a solid C substrate, their hydrological connections, effectiveness, and factors that limit their performance. The main denitrifying Keywords: bioreactors are: denitrification walls (intercepting shallow groundwater), denitrifying beds (intercepting Denitrification concentrated discharges) and denitrifying layers (intercepting soil leachate). Both denitrifcation walls and Bioreactor − Nitrate beds have proven successful in appropriate field settings with NO3 removal rates generally ranging from −3 −1 −3 −1 Effluent 0.01 to 3.6gNm day for walls and 2–22gNm day for beds, with the lower rates often associated with nitrate-limitations. Nitrate removal is also limited by the rate of C supply from degrading substrate − and removal is operationally zero-order with respect to NO3 concentration primarily because the inputs − − of NO3 into studied bioreactors have been generally high. In bioreactors where NO3 is not fully depleted, removal rates generally increase with increasing temperature. Nitrate removal has been supported for up to 15 years without further maintenance or C supplementation because wood chips degrade suffi- ciently slowly under anoxic conditions. There have been few field-based comparisons of alternative C − substrates to increase NO3 removal rates but laboratory trials suggest that some alternatives could sup- − port greater rates of NO3 removal (e.g., corn cobs and wheat straw). Denitrifying bioreactors may have a number of adverse effects, such as production of nitrous oxide and leaching of dissolved organic matter (usually only for the first few months after construction and start-up). The relatively small amount of field data suggests that these problems can be adequately managed or minimized. An initial cost/benefit analysis demonstrates that denitrifying bioreactors are cost effective and complementary to other agri- cultural management practices aimed at decreasing nitrogen loads to surface waters. We conclude with recommendations for further research to enhance performance of denitrifying bioreactors. © 2010 Elsevier B.V. All rights reserved. 1. Introduction trated discharges in agricultural systems occur through under-field tile drainage and ditches (Duda and Johnson, 1985; Dinnes et al., The nitrogen (N) cascade is an increasingly important global 2002) while diffuse sources are typically through the discharge of issue as N flowing through ecosystems has multiple impacts on shallow groundwater to surface waters. Nitrogen that is captured in terrestrial, aquatic and atmospheric environments (Galloway et al., biomass passes through the food chain and ends up in wastewater 2003, 2008). In agricultural systems, N in excess of plant and ani- streams, which are ultimately discharged to surface waters. mal needs can leach to shallow groundwater and ultimately enter A number of strategies are being implemented to reduce the N surface waters through concentrated or diffuse discharges. Concen- load to aquatic ecosystems. Sophisticated technologies have been developed to remove N from wastewater and are employed in municipal treatment plants and septic tank systems (Oakley et ∗ Corresponding author. Tel.: +64 7 858 3700; fax: +64 7 858 4964. al., this issue). In agricultural ecosystems, many land manage- E-mail address: [email protected] (L.A. Schipper). ment approaches have been proposed to reduce N losses, such as 0925-8574/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2010.04.008 Please cite this article in press as: Schipper, L.A., et al., Denitrifying bioreactors—An approach for reducing nitrate loads to receiving waters. Ecol. Eng. (2010), doi:10.1016/j.ecoleng.2010.04.008 G Model ECOENG-1647; No. of Pages 12 ARTICLE IN PRESS 2 L.A. Schipper et al. / Ecological Engineering xxx (2010) xxx–xxx improved N use efficiency in crops, managing N inputs through bioreactors (Figs. 1 and 2; Tables 1 and 2). Designs differ primarily − soil tests, land management plans and controlled drainage (Sims in the hydrologic connection between water containing NO3 and et al., 1995; Drury et al., 1996; Dinnes et al., 2002; Jaynes et C source and the ratio of source area:treatment area. Broadly, we al., 2004). Integrating wetland and riparian buffers into the agri- use terminology first used by Robertson and Cherry (1995). cultural landscape have also been demonstrated as a potentially “Denitrification walls” are where solid C material is incorpo- useful way to reduce N losses to surface waters (Hill, 1996; Kadlec, rated vertically into shallow groundwater perpendicular to the 2005). flow. Darcian flow, saturated hydraulic conductivity, hydraulic gra- The most widely understood process of permanent N removal dient and the flow paths intercepted by the walls controls the flux − from terrestrial and aquatic ecosystems is heterotrophic denitri- of NO3 into the walls. Walls may intercept natural groundwater − fication, which converts nitrate (NO3 ) to N gases using a carbon flow paths, or groundwater flow paths that have been altered by (C) source as the electron donor and for growth (Seitzinger et al., subsurface tile drainage systems or by the morphometry and rela- 2006; Coyne, 2008; Rivett et al., 2008). In this review, we focus on tively higher saturated hydraulic conductivity of C additions within heterotrophic denitrification but we recognise that other micro- the wall. The source area is roughly limited to the boundaries of the bial processes such as anaerobic ammonium oxidation (Anammox) wall orthogonal to the flow direction. While the term “wall” sug- and chemo-autotrophic denitrification can also produce N gases gests a barrier to flow, these walls are designed to sustain elevated (Burgin and Hamilton, 2007). The rates and controlling factors of hydraulic conductivities (i.e. >10 m day−1) conducive to substan- these microbial processes deserve further attention, but are not tial rates of shallow groundwater flow. Denitrification walls can covered in this review. be 100% wood chips (Farhner, 2002; Jaynes et al., 2008) or saw- At the microbial scale, the rate of heterotrophic denitrifica- dust mixed with soil (Robertson and Cherry, 1995; Schipper and − tion is controlled by concentrations of oxygen (O2), NO3 and C Vojvodic-Vukovic, 1998). (Seitzinger et al., 2006). The availability of a degradable C source “Denitrification beds” are containers (sometimes lined) that − − to drive denitrification becomes critical where NO3 is present are filled with wood chips and receive NO3 in concentrated dis- in excess, such as in many wastewater plants and agricultural charges either from a range of wastewaters (Robertson et al., settings. Aerobic microorganisms obtain energy through the oxi- 2005a; Schipper et al., this issue) or tile/drain discharge (Blowes dation of organic compounds, using O2 as the electron acceptor, et al., 1994; Robertson et al., 2009; Robertson and Merkley, until the environment becomes energetically favourable for the 2009). Denitrification beds have also been installed into existing − use of NO3 as an electron acceptor. As such, organic C plays two stream beds or drainage ditches and are specifically referred to key roles in promoting denitrification; firstly, to provide an anoxic as “stream bed bioreactors” (Robertson and Merkley, 2009). The environment and, secondly, to act as an electron donor for deni- source area:treatment area ratio for beds is usually much greater trification. In wastewater treatment systems, this C is part of the than in wall designs, due to either natural or artificial drainage waste stream, but can also be supplemented with liquid C sources networks that intercept and funnel groundwater inputs to the such as methanol (Oakley et al., this issue; Henze et al., 2008). In bioreactor. Beds, referred to as “upflow bioreactors” have also been agricultural soils, denitrification can be limited by sufficient labile adapted for use along streambanks (van Driel et al., 2006a,b). These C to create an anoxic environment and provide energy for denitri- upflow bioreactors do not receive input from a specific point